1. Field of the Invention
The present invention refers to a liquid developer for color electrophotography and to a process for its preparation.
2. Description of the Prior Art
Production of colored or multi-colored images by electrophotographic processes can be accomplished, for example, as disclosed in U.S. Pat. No. 3,901,696, of which we are the inventors and which is assigned to the same assignee. According to this patent, an image is obtained by exposing a photoconducting layer to an original, thus producing on or in this layer a conductivity pattern corresponding to the original. The pattern is used to control a field produced by means of two electrodes, the field being in the space between the photoconducting layer and a support for the image to be produced. Electrically-charged toner particles in the above-mentioned space are moved through this field, the toner particles having their origin in a liquid developer introduced into the space between the photoconducting layer and the image support prior to establishing the field. The polarity of the toner particles and the polarity of the electrode assigned to the photoconducting layer are either selected opposite to one another so that the toner particles deposit on the photoconducting layer, or these polarities are selected to be identical so that the toner particles deposit on the support. The electrical field is subsequently interrupted, as a result of which in the first case toner particles migrate from the photoconducting layer to the support and produce on the latter a positive image corresponding to the original whereas in the second case, toner particles migrate from the support to the photoconducting layer and thus leave behind on the support a negative image corresponding to the original.
The present invention thus concerns a liquid developer for color electrophotography, particularly the process (hereinafter referred to as the "conductivity modulation process") of the above-cited U.S. patent, and while it is possible, of course, to produce color images electrophotographically according to the mentioned process using already known developers, the images produced in that manner still leave much to be desired with respect to contrast, sharpness, resolution and color saturation. Furthermore, in the production of multicolored images by superposition of several partial color images, certain difficulties have been encountered, for example, when conventional "self-fixing" agents in known developers deposit together with color pigments of a partial image and modify the surface of the support bearing the partial image. The modification occurs in such manner that it significantly disturbs the subsequent step for formation of a next partial color image, with resultant inability to produce high quality copies of the original.
The above-mentioned difficulty cannot be eliminated simply by omission of the fixer in the developer since other essential properties of the developer are than impaired unacceptably. Another requirement with respect to the dye pigments of such developers lies in the fact that the images produced therewith must be permanent, and light- and solvent resistant. Further, in known developers the color pigments are customarily embedded in a resin shell. Such a dye pigment resin mass must be pulverized to a fineness not possible by mechanical means as regards the pigments used in the developer disclosed here.
Since no optimal developer is yet known for the above-mentioned process for electrophotographic image production, the basic task of the present invention is to create a liquid developer for production of colored or multicolored images by means of electrophotography; a developer which does not exhibit the named disadvantages and is particularly characterized by the fact that it results in high contrast, sharp color images exhibiting a high image resolution and a high color saturation, one which ensures the unobjectionable superposition of individual partial color images in the production of multicolored copies. The task also comprises creation of a process for production of the named liquid developer.